In recent years, a portable communication device has made outstanding development, and particularly cellular phones are under rapid reduction in size, weight and thickness. Accordingly, an antenna positioned in the cellular phones and the like has come closer to a human head and other electronic devices susceptible to noise. Therefore, an interaction of the antenna with the human head or other electronic devices has become a subject of discussion.
Radio waves radiated from the antenna of the cellular phone are partially absorbed by the closest human head and the remaining radio waves are radiated to the air. Absorption of electromagnetic energy by the human head might cause a problem that the radiation efficiency and communication characteristics of the antenna are deteriorated. When the cellular phone is used, the antenna is close to the head, which is locally exposed to a strong electromagnetic field, so that adverse effects on a human body due to an increase in amount of power partial-body absorption are worried about. Therefore, guidelines (an amount of power partially absorbed per unit mass: SAR (Specific Absorption Rate)) for an electromagnetic energy partial-body absorption directed to the cellular phone have been set up in the U.S., Europe and Japan, consecutively.
Under the circumstances described above, the portable communication device represented by the cellular phone is desired to reduce the amount of electromagnetic energy absorbed by the human body (e.g., an exposure amount of the human head to electromagnetic energy). The exposure to the electromagnetic energy can be reduced by sufficiently increasing the distance between the antenna and the human body, but it is not a realistic solution to solve the problem because the cellular phone cannot be used if it is separated from the ear by a sufficient distance.
As a technology to decrease the level of an electromagnetic field generated around the antenna, it is known to dispose as an electromagnetic wave absorber a composite magnetic material containing soft magnetic powder and an organic binder or an inorganic binder at the antenna base or the like of the cellular phone (see, for example, Patent References 1 and 2). Here, the electromagnetic waves are consumed as heat loss by utilizing the fact that an imaginary component μ″ of a complex relative permeability μ of the composite magnetic material rises steeply near the transmit frequency of the antenna.
Patent Reference 3 describes a granular magnetic film, which has ferromagnetic fine grains and a grain boundary substance formed of a metal oxide, a metal nitride or the like and arranged around them, as an electromagnetic wave absorption film which has the imaginary component μ″ of the complex relative permeability μ in a gigahertz range enhanced. Patent Reference 4 describes an electromagnetic wave absorber that the real number component μ′ and the imaginary component μ″ of the complex relative permeability μ in a high-frequency region of 1 to 3 GHz meet the relationship of μ′>μ″.
Among the above electromagnetic field level reducing technologies, the technology which consumes the electromagnetic waves as a heat loss based on the imaginary component μ″ of the complex relative permeability μ of the magnetic material has a disadvantage that the strength of a transmission signal itself lowers because of the reduction to the electromagnetic field level near the antenna. Patent Reference 1 describes that as a SAR suppressor application condition that the imaginary component μ″ value of the complex relative permeability μ of the electromagnetic wave absorber is increased and a tan δ (=μ″/μ′) value is also increased.
For a conventional electromagnetic wave absorber that the imaginary component μ″ of the complex relative permeability μ is larger than the real number component μ′, a loss due to energy absorption cannot be disregarded. Therefore, it is necessary to find optimum values of an electromagnetic wave absorber adhering position, area and the like by experiments so that an energy loss can be minimized, namely an output drop of the electromagnetic waves radiated from the antenna is minimized. Therefore, a plan for improving the SAR suppression effect is merely described, and useful SAR measures have not been provided.
The electromagnetic wave absorber described in Patent Reference 4 suppresses a loss of electromagnetic waves by determining that the μ″ having functions to absorb and attenuate the electromagnetic waves is decreased to be smaller than the μ′ which does not contribute to the loss but relates to a magnitude of the complex relative permeability μ. But, since a noise frequency is generally higher than the signal frequency, it is necessary to increase a resonant frequency sufficiently. And, fine particles have the shape anisotropy dispersed, and their aggregate, namely an electromagnetic wave absorber, becomes easy to allow the passage of a magnetic flux in a vertical direction. Therefore, there is a possibility that the magnetic flux is directly leaked through the component inserted between the human body and the antenna from the antenna to the human body via the interference preventing component.
[Patent Reference 1] JP-A 2002-158484 (KOKAI)
[Patent Reference 2] JP-A 2002-158488 (KOKAI)
[Patent Reference 3] JP-A 2002-158486 (KOKAI)
[Patent Reference 4] JP-A 2004-128001 (KOKAI)